The present invention relates to a belt-type fixing device that is used in an electrophotographic image forming apparatus.
In Japanese Patent Laid-Open Publication HEI 08-334997 has been disclosed a belt-type fixing device 70 shown in FIG. 4. The belt-type fixing device 70 has an endless-sheet-like fixing belt 72. The fixing belt 72 is wound around a heating roller 76 having a heater lamp 74 as a heat source therein and around a fixing roller 78 having an elastic layer on an outer circumference thereof. A pressurizing roller 80 that is driven to rotate in a direction of an arrow C is in pressure contact with the fixing roller 78 with the fixing belt 72 interposed between, and contact part between the pressurizing roller 80 and the fixing belt 72 forms a fixing nip. A donor roller 82 is in pressure contact with an outer surface of the fixing belt 72 between the heating roller 76 and the fixing roller 78. The fixing belt 72 is pressed inward by the donor roller 82, and a contact surface between the fixing belt 72 and the heating roller 76 is thereby enlarged stably, so that heat transfer from the heating roller 76 to the fixing belt 72 is efficiently effected.
Any of the heating roller 76, the fixing roller 78, and the donor roller 82 can be rotated. The pressurizing roller 80 is driven to rotate in the direction of the arrow C, and the fixing belt 72 is thereby rotated in a direction of an arrow D. A thermistor 84 that is temperature detecting means is provided so as to be in contact with an outer circumference of the heating roller 76. By on-off control over the heater lamp 74 on basis of temperatures detected by the thermistor 84, temperatures of the heating roller 76 and the fixing belt 72 can be kept at specified values.
In the belt-type fixing device 70, however, a tension in the fixing belt 72 is increased and a driving torque for the pressurizing roller 80 is concomitantly increased. As a result, stable conveyability with the fixing belt 72 is not easy to achieve.
Provided a nip forming member that is fixed so as not to rotate is used in place of the rotatable fixing roller 78, particularly, a tension in the fixing belt 72 is further increased by sliding friction of an inner surface of the fixing belt 72 on the nip forming member. In this case, an excessive load for driving the fixing belt 72 tends to cause slip between the fixing belt 72 and the pressurizing roller 80, so that it becomes difficult to achieve stable conveyability with the fixing belt 72. With a reduction in the tension in the fixing belt 72, on the other hand, a frictional force against the nip forming member is decreased and a load for driving the fixing belt 72 is thereby decreased. Without provision of the donor roller 82, however, a force of pressing the fixing belt 72 against the heating roller 76 is lost, and it is therefore necessary to stabilize an area of the contact surface between the fixing belt 72 and the heating roller 76.